In the telecommunications field, it is often desirable and frequently necessary to ascertain certain transmission characteristics, such as relative group delay, modem linearity, and differential phase and gain, of a microwave radio communication link (MRCL). In operating and maintaining a radio system at its optimum performance, the transmission characteristics, and particularly group delay slope distortion, must be occasionally measured so that various corrective adjustments can be periodically made. For example, fine tuning of RF or IF (Radio Frequency or Intermediate Frequency) filter delay equalizers can usually be made to correct for small changes in group delay slope in the transmission frequency band.
Most of the practical methods which are employed today to make field measurements of radio transmission characteristics use commercially available transmission test sets. These test sets simplify the testing procedures and also reduce the time required for performing the measurements. Typical of such commercial test sets are the GTE Italia CSM Base 221-C/222-C Radio Link Test Set (Milan, Italy), the Hewlett-Packard 3710/3702 Link Analyzer (Palo Alto, Calif.), and the Siemens K1005/K1046 Sweep Frequency Test Sets. Group delay distortion and the related transmission characteristics are measured by these test sets using the well-known Nyquist method of measuring group delay, which method uses two signals. This test method will hereinafter be called "the two-tone test method". This method applies two separate test frequencies in the base band frequency group through the FM radio link under test. These two test frequencies are generally referred to as the sweep frequency and the search frequency (or modulating frequency). The sweep frequency is typically a very low frequency signal, in the order of 16 Hz to 100 Hz, and may be a sinusoidal or triangular wave form. It is applied to the baseband input of the radio at a high level, causing the frequency modulator in the radio to deviate the FM carrier frequency over the frequency band of interest. Added to the sweep frequency is a low-level search frequency, which is an intermediate frequency in the radio baseband frequency group. 278 KHz is typically used as the test search frequency. This generally accepted test method for measuring group delay distortion and the related transmission characteristics is covered in detail in the Fifth Plenary Assembly (CCITT, December 1972, Vol. IV.2, "Maintenance," published by the International Telecommunications Union, 1973, pp. 492-507).
While such test sets can be used to perform the required testing of group delay, the disadvantages are the requirement for additional, expensive test equipment and the problem of having the test equipment at the transmitter and receiver ends of the system. These disadvantages are overcome by having the facility built in at a relatively nominal cost.